Since about 1987, in particular, there has been much published about semiconductor lasers that are capable of emitting light within the 600 nm to 700 nm wavelength range for use in many applications, such as optical disc apparatus, laser printers, bar code readers and the like. An example of an earlier article on this subject is the paper of K. Kobayashi et al. entitled, "AlGaInP Double heterostructure Visible Light Laser Diodes with a GaInP Active Layer Grown By Metalorganic Vapor Phase Epitaxy", IEEE Journal of Quantum Electronics, Vol. QE-23(6), pp. 704-711, June 1987. These devices include ternary and quaternary Group III-V materials including In, P or InP for achieving visible lightwave generation. The devices have an active region of GaInP or AlGaInP lattice matched or coherently strained to a GaAs substrate with cladding layers of AlGaInP and confinement layers of AlInP. The active region may be a single quantum well of GaInP or multiple quantum well structure of GaInP well layers and AlGaInP barrier layers or alternating layers of AlGaInP of different compositional ratio. A more recent publication is U.S. Pat. No. 5,144,633 to Ohnaka et al. which is discloses a visible wavelength, semiconductor laser device having an active region of GaInP lattice matched to a GaAs substrate, with cladding layers of AlGaInP and at least one confinement layer of AlInP. A stopper layer of GaInP (doped or undoped) is usually formed within the AlInP confinement layer for aiding in an etching operation to form a buried or inner loss-guide stripe region through a subsequently formed current blocking layer of GaAs.
A problem in these devices is the confinement of carriers to the active region and provision for a low resistance path for carrier supply to the active region. This problem is addressed, in part, in U.S. Pat. No. 5,274,656 to Yoshida. In this patent, reference is made to the fact that higher Al composition ratios in the cladding layers are preferable for efficient confinement of carriers to the active region. However, it happens that such higher Al composition ratios bring about more heat generation affecting the long term reliability of these devices. Attempts to decrease the resistivity of the cladding layers through decrease of the layer resistivity through increase the doping level of the layer is not effectual for AlGaInP layers, for example, because the doping activation ratio level is reduced as Al content increases. In order to achieve shorter wavelengths into the visible spectrum, the bandgap of the active layer can be increased, but the difference in bandgap between the active region and the cladding region becomes closer, decreasing the carrier confinement to the active region. Yoshida provides an upper cladding layer comprising AlGaInP that decreases in Al composition ratio from its inner most limit closest to the GaInP active region to its outer most limit. The overall Al composition ratio is lowered so that carrier density in at least the outer reaches of the cladding layer is increased without need of increasing the layer doping level. However, further reductions in forward voltage drop are desired in cladding layer areas of these devices, particularly in the case where narrow pumping stripes are employed with broad area beam output with higher output power.
Thus, the problem still persists on how to further reduce the voltage drop in these cladding layers to provide a low resistance path to the active region without sacrificing high carrier confinement to the active region. This is particularly important in visible wavelength, semiconductor devices that are designed to provide a high level of power, such as employing on the same semiconductor chip or on a different semiconductor chip, a single mode section and a gain section for achieving high power. One such device comprises a master oscillator in combination with a beam enlarging or diverging gain section providing a beam diverging phase front forming a stable oscillator. Another such device comprises a single mode section and a diverging gain section utilizing a beam diverging phase front forming an unstable oscillator. Such devices are disclosed in the U.S. Pat. Nos. 5,539,571 and 5,537,432, which patents are assigned to the assignee herein and are incorporated herein by their reference. These devices demand higher carrier concentration and carrier supply to a comparatively narrow stripe region (e.g., in the range of about 3 .mu.m to about 5 .mu.m wide) compared to the broad diverging gain pumping region and requiring good carrier conversion efficiency in the single mode section through enhanced carrier supply. One manner of accomplishing good carrier conversion efficiency is to provide a wider pumping stripe for the single mode section, but a wider pumping stripe means a larger aperture into the diverging gain region which can result in poor beam formation and divergence.
What is needed for these combination single mode and beam enlarging gain resonator devices is to enhance the carrier density through a decrease in the layer voltage drop to improve the conversion efficiency of carriers in the single mode section while providing good beam divergence into the beam diverging gain section providing an improved flattening and broadening of the Gaussian beam profile.
It is, therefore, a primary object of this invention to provide a visible wavelength, semiconductor optoelectronic device with high CW power, diffraction limited, visible beam.
It is another object of this invention to provide good beam divergence with improved flattening and broadening of the Gaussian beam profile with enhancement to beam edges by permitting the beam to initially expand before more full and aggressive pumping is applied in wider regions of the beam diverging gain section with accompanying high carrier conversion efficiency in a stable or unstable resonator light emitting devices having a single mode section and a beam diverging gain section.
It is a further object of this invention to improve the quality of growth of sensitive Group III-V, AlGaInP-containing materials employed in the active region and confining and cladding layers of visible wavelength, semiconductor optoelectronic devices.
It is a still further object of this invention to improve the formation and utility of high resistance regions in forming beam diverging gain sections employed in high power, visible wavelength, semiconductor optoelectronic devices.